Gears used in drive trains which rotate at relatively high speeds, e.g., automotive and aircraft drive trains, must be manufactured with great precision. Such precision usually requires a multi-step manufacturing process in which the gear is first cut to produce teeth which are very close to the final shape desired, then removed from the cutting machine and heat-treated to harden the surface of its teeth, and thereafter placed in a finish-cutting or grinding machine to form the hardened teeth into the precise shape required, namely, with appropriate curvature and with the spacing between each pair of teeth being uniform around the gear.
The heat treating process causes the body and teeth of the gear workpiece to become distorted, resulting in tooth-to-tooth variations which must be removed during the finishing process. Also, seating and positioning of the workpiece on the work spindle varies not only from machine to machine, but also between each mounting on the same machine. These variations in positioning and seating result in the radial and/or axial displacement of the axis of the gear workpiece from the axis of the work spindle, causing an asymmetrical variation in flank-to-flank spacings of the gear teeth as they are measured around the gear relative to the position of the finishing tool. This asymmetrical spacing variation is referred to as "runout." In order to produce a precision gear, the finishing tool must be able to remove all such heat-treat distortion and runout variation. Therefore, it is essential that the partially-processed gear be positioned relative to the finishing tool (e.g., grinding wheel) in a manner so that each flank of the workpiece will be contacted and shaped by the tool during the finishing process.
Of course, prior to any final machining, a gear-shaped workpiece must be appropriately "stock divided," i.e., the teeth of the partially finished gear must be positioned properly relative to the tool prior to the initiation of the finishing cycle. Traditionally, such stock division has been accomplished manually by skilled operators or, sometimes, automatically by mechanical means, such as, by inserting a ball-shaped probe into a tooth space of the workpiece gear to rotate it into an appropriate position relative to the grinding wheel. More recently, with the advent of sophisticated electronic equipment, stock division is being performed with the assistance of very sensitive contact or proximity (non-contact) probes which accurately measure the relative position of the flanks of the rough-cut gear teeth, such measurements being delivered to appropriate electronic computing apparatus where it is stored, processed, and used to generate control signals for automatically setting the relative position of the tool and workpiece prior to the initiation of the machining operation.
As measuring equipment has become more sophisticated and accurate, so has the process of stock division. In one known method, following the mounting of the work gear on the machine tool, but prior to initiating the final grinding process, a machine-mounted contact probe is moved into the space between two adjacent teeth of the work gear. The work gear is then rotated in one direction until the flank of one of the adjacent teeth activates the probe, and the angular position of the work gear is then registered in a computer. Next, the gear is rotated in the opposite direction until the flank of the adjacent tooth activates the probe, and the angular position of the gear is again registered in the computer. The difference between these two angular positions is then computed, and then the gear is rotated under computer control back half the distance toward the first-measured flank. At this point, the position of the probe precisely indicates the center of the tooth slot. The grinding wheel is then positioned relative to the work gear based upon this information.
However, as indicated above, the distortions caused by heat treatment are not uniform, and so in order to achieve more precise stock division, the prior art process just described above is repeated in a plurality of tooth slots located around the work gear, and the successive measurements are combined to compute an average value which is then used to determine the initial position of the grinding wheel relative to the gear.
The precision of this prior art stock division process varies directly with the number of tooth spaces measured by the probe, the highest accuracy being attained when all of the tooth spaces are measured and averaged for the final computation. Of course, the more tooth spaces probed, the longer it takes to complete such stock division, and, in order to reduce production costs, it is desirable to minimize the number of spaces being measured. However, because the flanks of the workpiece teeth may have very irregular errors due to heat treat warping, the accuracy of the finished part may be seriously affected if the particular flanks randomly selected for measurement do not include the "worst case" tooth space.
Other prior art stock division systems use a variety of different means for measuring the flanks of the workpiece. Some compare the workpiece to a correctly dimensioned reference part, i.e., a "master" gear, which is first mounted in the machine tool, each of its tooth spaces being measured with a probe or, in some systems, measured by a geared wheel which intermeshes with the master gear and either rotates the master gear or is rotated by it. These master gear measurements, (e.g., the spacings between the successive teeth of the master gear as measured by the probe, or the pulses generated by the geared measuring wheel as it rotates with the master gear) are stored in a computer and, thereafter, compared to similar measurements made with each workpiece, the initial position of the grinding wheel relative to the workpiece being selected in accordance with the differences between the master gear information and that measured on the workpiece.
While there are many known prior art systems for stock dividing gear-shaped workpieces, there remains a definite need for improvement, namely, a system which not only positions the workpiece so that it can be properly and precisely finished, but one that also accomplishes this goal in a relatively fast and, therefore, less costly manner.